The present invention relates to the reconstitution or restoration of images from samples distributed over a two-dimensional frame and each represented by a quantified generally binary signal, and to the reproduction of images at a remote location by scan analysis of the original image, sampling, coding, transmission over any channel (typically over a telephone line) and reconstitution at the remote location.
More precisely, methods for reproduction of documents at a remote distance are known in which successive rows of the original document are sampled for generating a large number of image elements, each of which has identifying coordinates x and y in a frame and is associated with a binary digit indicating if it must be considered as "black" or "white".
Each sample being located at random manner with respect to the pattern borne by the document (lines of a drawing or alphanumerical characters), each sample has to be processed for determining whether a binary "0" or "1" is to be applied thereto, according to the value of its mean luminance, by comparison with the range of the luminances over the whole of the document. The binary discreteness (decision on the luminance level) and spatial discreteness (due to the random localization of the sample) detrimentally affect the message data from the beginning and correspond to an alteration of the contours. The smaller the image element, the weaker the degradation. An analysis with eight image elements per millimeter provides a satisfactory discreteness in the case of typed texts. As a counterpart, the volume of data to be transmitted is extremely large. With four elements per millimeter, the volume of data is divided by 4, but the image is so degraded that after reconstitution the text becomes difficult to read and there are ambiguities between various figures and letters.
It is an object of the invention to provide an image reconstitution method which reconstitutes a higher number of elements than that which has been transmitted by using the correlation between adjacent elements and correlatively improves the resolution.
According to an aspect of the invention, there is provided a method for reconstitution of an image from samples N.sub.o,o, N.sub.2,o, . . . , N.sub.2p,o, . . . , N.sub.o,2, . . . , N.sub.o,2q, . . . N.sub.2P,2q, . . . distributed over a two-dimensional matrix and each represented by a quantified signal, wherein four samples m.sub.2p,2q, m.sub.2p+1,2q, m.sub.2p,2q+1, m.sub.2p+1,2q+1 are reconstituted from the samples N.sub.2p,2q and the like according to the expansion algorithm: ##EQU2## and the quantified signals m are applied to a reproduction apparatus having a frame homothetic with the frame defining samples N. The samples are typically quantified at two levels, 0 and 1. Then the "+" and ".times." symbols should be understood as designating the conventional boolean (rather than arithmetic) operations. It should further be understood that the above algorithm may be written in other equivalent forms, as will be seen later. The reconstitution method is applicable to any message consisting of data obtained by analysis with low resolution. However, it is particularly advantageous in transmission methods in which there is successive compression of data obtained by high resolution analysis, then transmission of the compressed data whose degree of cross-correlation has been reduced by the compression operation. The reconstituted image than has a resolution which is approaching that obtained by direct printing of the original message generated. during the analysis operation.
The original image will frequently be analysed in horizontal and vertical directions, since such directions are statistically frequent in documents bearing alphanumeric indications. However, the scanning frame can have two differently oriented directions.
According to a second aspect of the invention, there is provided a reproduction method in which a reconstitution method as defined above is applied to signals received over a transmission line and originating from a data compression coding method. During the compression, the samples N which will be transmitted are generated from initial samples n.sub.o,o, n.sub.1,o, n.sub.2,o, . . . , n.sub.p,o, . . . , n.sub.p,q, n.sub.p+1,q, . . . distributed over a two-directional frame, according to the algorithm: EQU N.sub.2p,2q =[n.sub.2p,2q .times.n.sub.2p+1,2q ]+[n.sub.2p,2q+1 .times.n.sub.2p+1,2q+1 ]+[n.sub.2p,2q .times.n.sub.2p,2q+1 ]+[n.sub.2p+1,2q .times.n.sub.2p+1,2q+1 ]
According to an other aspect of the invention, image reproduction and reconstitution systems adapted to carry out the abovementioned methods are provided. Such systems can be constituted using simple logic circuits only. Whatever the embodiment used, the reconstitution is effected with accentuation and smoothing along the two directions of the frame, which increases the legibility of texts and renders the reproduction of graphs less statistically dependent on the sampling spatial "phase", that is on the position of the sampled elements with respect to the characters.
The invention will be better understood from the following description of particular embodiments given by way of examples. The description refers to the accompanying drawings.